Single phase induction motors have been widely used due to their construction simplicity, rigidity and reliability. They are useful for low power range applications, such as hermetic refrigeration compressors, washing machine motors, pumps and some industrial applications.
Single phase induction motors are essentially composed of a cage type rotor and a stator winding, basically two windings, one for the main winding and the other for the starting winding.
When an alternating voltage source is applied only to the main winding of a single phase induction motor, a magnetic field fixed in space and alternating in amplitude is created, but which does not develop any torque to start the rotor movement. Thus, it is necessary to create a rotating magnetic field to take the rotor from the static condition, so that the starting is initiated.
Such rotating field is achieved by supplying the starting winding with a current which is phase-displaced in relation to the current flowing in the main winding and at an angle as near as possible to 90.degree..
The phase displacement of the current in the starting winding in relation to the current in the main winding, can be achieved by construction differences, such as by designing the starting winding with a power factor higher than that of the main winding, or by inserting an external impedance connected in series to the starting winding, the impedance usually being a resistor or a capacitor.
Therefore, it is necessary to install a device in series with the starting winding circuit which temporarily connects this circuit to the supply source, until the rotor has reached a speed near to the synchronous speed, thereby giving rise to the starting.
In motors in which a higher efficiency is required, the starting winding is not fully disconnected from the supply source after the starting, with a capacitor being mounted in series with this winding. This running capacitor, or permanent capacitor allows the flow of a low value current, keeps a double phase feed condition for the motor, increases its torque and substantially increases its efficiency.
For motors having such a configuration, using a permanent capacitor, there is shown a type of starting device as described in Brazilian patent document PI 201.210 consisting of a resistor having a positive temperature coefficient (PTC). This device, which is connected in series to the starting winding, has a low resistance at ambient temperature, thereby allowing the passage of a high current during the starting and which, after a predetermined time and due to a self-heating effect, presents a very high resistance, thereby practically acting as an open circuit.
During normal motor running, the voltage on the PTC is high, keeping it heated and dissipating an amount of power varying from 1.5 to 5 W or higher, depending on the ambient temperature and constructive form. Such power expended by the PTC during the entire motor running period decreases the motor efficiency, thus being undesirable.
Another known starting device is a current relay which keeps the starting coil circuit on, while the current in the main winding is high. Although it is a very simple device and practically does not consume any power during the normal motor running period, it has the disadvantage of not being useful in configurations having a permanent capacitor. This fact is due to the delays inherent to relays which cause their contacts to close when the permanent capacitor has already stored a certain amount of energy, and this energy will be dissipated over the contacts of the relay at the time it is closed, thereby quickly causing relay contact destruction.